Sahih Bukhari Volume 007, Book 067, Hadith Number 427.
Narrated By Zahdam : We were in the company of Abu Musa Al-Ash'ari and there were friendly relations between us and this tribe of Jarm. Abu Musa was presented with a dish containing chicken. Among the people there was sitting a red-faced man who did not come near the food. Abu Musa said (to him), "Come on (and eat), for I have seen Allah's Apostle eating of it (i.e. chicken)." He said, "I have seen it eating something (dirty) and since then I have disliked it, and have taken an oath that I shall not eat it ' Abu Musa said, "Come on, I will tell you (or narrate to you). Once I went to Allah s Apostle with a group of Al-Ash'ariyin, and met him while he was angry, distributing some camels of Rakat. We asked for mounts but he took an oath that he would not give us any mounts, and added, 'I have nothing to mount you on' In the meantime some camels of booty were brought to Allah's Apostle and he asked twice, 'Where are Al-Ash'ariyin?" So he gave us five white camels with big humps. We stayed for a short while (after we had covered a little distance), and then I said to my companions, "Allah's Apostle has forgotten his oath. By Allah, if we do not remind Allah's Apostle of his oath, we will never be successful." So we returned to the Prophet and said, "O Allah's Apostle! We asked you for mounts, but you took an oath that you would not give us any mounts; we think that you have forgotten your oath.' He said, 'It is Allah Who has given you mounts. By Allah, and Allah willing, if I take an oath and later find something else better than that. then I do what is better and expiate my oath.'"